1. Field of the Invention
The present invention relates to a Time Division Duplex (TDD) switch of a TDD wireless communication system. More particularly, the present invention relates to an apparatus for protecting a receiver when a high-power transmission signal is introduced to the receiver out of sync due to erroneous operations such as a malfunction of the TDD switch or when power of the TDD switch is blocked. In addition, the present invention relates to a TDD switch using a transmission line and a transmission line stub without having to use a conventional circulator.
2. Description of the Related Art
In general, a Time Division Duplex (TDD) wireless communication system uses a TDD switch to change its mode between a transmission mode and a reception mode. Such a mode change allows a transmission path to be separated from a reception path, so that a receiver can be protected when in the transmission mode. The TDD switch operates in response to a TDD control signal of the wireless communication system. The TDD switch is generally located as will now be described.
FIG. 1 illustrates a conventional TDD switch in a TDD wireless communication system.
Referring to FIG. 1, a TDD switch 107 is connected to a Power Amplifier (PA) 103, an antenna 111, and a Low Noise Amplifier (LNA) 115.
When the wireless communication system operates in the transmission mode, a transmission signal transmitted from a transmitter 101 is amplified to a high-power transmission signal by the PA 103 and is then radiated through the antenna 111 via a transmit port 105 and an antenna port 109. In this cases the TDD switch 107 operates in the transmission mode and thus isolates the transmitter 101 from a receiver 117. Therefore, the receiver 117 can be protected against the high-power transmission signal from the transmitter 101.
When the wireless communication system operates in the reception mode, a reception signal received through the antenna 111 is transmitted to a receive port 113 via the antenna port 109. In this case, the TDD switch 107 operates in the reception mode and thus enables the reception signal to be transmitted to the receive port 113. The reception signal itself has a significantly low power level due to attenuation and noise. Therefore, the reception signal is amplified by the LNA 115 in which a signal is amplified with minimum noise. The amplified reception signal is transmitted to the receiver 117.
FIGS. 2A and 2B illustrate a conventional TDD switch having a circulator and a λ/4 transmission line.
FIG. 2A illustrates a conventional TDD switch having a circulator 201 and a λ/4 transmission line 202. In FIG. 2A, the λ/4 transmission line 202 and a pin diode 203 are connected in three connection configurations. About 20 dB of signal attenuation can be prevented per each connection configuration. Thus, the three connection configurations shown in FIG. 2A can prevent about 60 dB of signal attenuation. The connection configurations are located between a receive port 206 and the circulator 201.
In the TDD communication transmission system, it will be assumed hereinafter that a transmit port 204 includes a PA, and the receive port 206 includes an LNA. An antenna is connected to an antenna port 205 of the TDD switch.
An isolator 207 transmits a signal only in one direction and is located between the transmit port 204 and the circulator 201. The isolator 207 is designed to pass only a transmission signal transmitted from the transmit port 204. Furthermore, the isolator 207 acts as a terminator for an introduced signal.
For example, when the transmission signal is not successfully radiated through the antenna and is thus reversely introduced, the isolator 207 terminates the introduced signal. Thus, the circuit of the transmit port 204 is protected.
The circulator 201 is a 3-port circuit element for branching signals. A resonance plate and a magnetic substance (e.g., ferrite) are placed inside the circulator 201 having a shape in which three ports are arranged by 120 degrees. The circulator 201 leads to an approximately 0.3 dB path loss when a power signal is transmitted in a direction from the isolator 207 to the antenna port 205. Also, the circulator 201 attenuates the power signal by a specific level (about 20 dB) in another direction from the circulator 201 to the receive port 206.
For example, when a TDD control signal operates in the transmission mode, the transmission signal amplified through the transmit port 204 exhibits an approximately 0.3 dB path loss while passing through the circulator 201 and is then radiated through the antenna via the antenna port 205. In the direction from the isolator 207 to the pin diode 203, the transmission signal is attenuated by a certain level (about 20 dB). The receive port 206 may be damaged when the attenuated transmission signal is introduced to the receive port 206.
The TDD control signal is used to control the transmit port 204 and the receive port 206 of the TDD wireless communication system. In response to the TDD control signal, the transmit port 204 amplifies the transmission signal and then radiates the amplified power signal to the antenna.
In addition, the TDD control signal is used to control a bias circuit 209 which regulates a Direct Current (DC) bias supplied to the pin diode 203. The DC bias is supplied to the pin diode 203 and is independent from wireless communication characteristics. The pin diode 203 acts as a part of the TDD switch according to the DC bias.
Although not shown in FIG. 2A, a capacitor is disposed between the circulator 201 and the λ/4 transmission line 202 so as to block the DC bias. Thus, the DC bias is prevented from being introduced to other circuits. Hereinafter, the capacitor for blocking the DC bias is assumed to be present throughout the figures.
According to transmission line theory, when the output port of the transmission line is open to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo cot βl. When the output port of the transmission line is shorted to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo tan βl. When the output port of the transmission line is connected to a 50 ohm transmission line, the impedance of the input port of the transmission line is expressed as Z=Zo=50 ohm. Here, β=2π/λ, and l is the length of the transmission line. As is known, waves have the same amplitudes at λ/4, 3λ/4, 5λ/4, 7λ/4, and so on. Hence, the λ/4 transmission line 202 may be generalized as a (λ/4)*(2m+1) transmission line [m=0, 1, 2, 3, . . . ]. The λ/4 transmission line 202 corresponds to a (λ/4)*(2m+1) transmission line [m=0, 1, 2, 3, . . . ], where m is 0.
The λ/4 transmission line 202 nearest to the receive port 206 is connected to the receive port 206. A nominal impedance of the λ/4 transmission line 202 is 50 ohm.
When the forward DC bias is supplied to the pin diode 203, the impedance of the pin diode 203 decreases. Thus, the impedance viewed from the λ/4 transmission line 202 towards the pin diode 203 becomes similar to a state of being connected to ground. In addition, when the impedance of one end of the λ/4 transmission line 202 decreases, according to the above expression of Z=−jZo tan β
where β=2π/λ, and l=(λ/4)*(2m+1)[m=0, 1, 2, 3, . . . ], the impedance Z of the other end of the λ/4 transmission line 202 becomes nearly infinite (open-circuited).
Conversely, when the reverse DC bias is supplied to the pin diode 203, the impedance of the pin diode 203 increases. As a result, the impedance viewed from the λ/4 transmission line 202 towards the pin diode 203 becomes nearly 50 ohm since it is a parallel impedance between an infinite impedance (open-circuited) and a 50 ohm impedance. Specifically, when the impedance of the pin diode 203 increases, the λ/4 transmission line 202 is substantially connected only to the 50 ohm transmission line. Thus, according to the above expression of Zo=50 ohm, the impedance Z of the other end of the λ/4 transmission line 202 becomes nearly 50 ohm. This is similar to the case when the circulator 201 is directly connected to the receive port 206.
Consequently, impedance changes in the pin diode 203 in response to the DC bias allow the output port of the λ/4 transmission line 202 to become substantially shorted to ground or substantially connected only to the 50 ohm transmission line.
In the transmission mode, when the TDD control signal is transmitted to the bias circuit 209, the bias circuit 209 supplies a forward DC bias to the pin diode 203. The forward DC bias allows the impedance of the pin diode 203 to become nearly 0 (short-circuited). Therefore, the λ/4 transmission line 202 is substantially connected to ground. According to the characteristic of the λ/4 transmission line 202, the impedance of an input port of the λ/4 transmission line 202 changes to be opposite to the impedance of an output port of the λ/4 transmission line 202 and thus becomes nearly infinite (open-circuited).
Accordingly, the transmission signal transmitted from the isolator 207 to the circulator 201 is reflected, thereby protecting the receive port 206 against the transmission signal.
In the reception mode, when the TDD control signal is transmitted to the bias circuit 209, the bias circuit 209 supplies a reverse DC bias to the pin diode 203. The reverse DC bias allows the impedance of the pin diode 203 to become nearly infinite (open-circuited). Therefore, the λ/4 transmission line 202 is substantially directly connected to the receive port 206. In this case, the impedance of the output port of the λ/4 transmission line 202 becomes 50 ohm, and the impedance of the input port of the λ/4 transmission line 202 also becomes 50 ohm. Accordingly, most of the transmission signal can be transmitted from the antenna port 205 to the receive port 205 via the circulator 201.
FIG. 2B illustrates a conventional TDD switch having a circulator 211 and a λ/4 transmission line 212. In FIG. 2B, the λ/4 transmission line 212 and a pin diode 213 are connected in two connection configurations. About 20 dB of signal attenuation can be prevented per each connection configuration. Thus, the two connection configurations shown in FIG. 2B can prevent about 40 dB of signal attenuation. The connection configurations are located between a receive port 216 and the circulator 211. The operation of the TDD switch of FIG. 2B is the same as that of FIG. 2A. Also illustrated in FIG. 2B are elements similar to those in FIG. 2A such as a transmit port 214, an antenna port 215, an isolator 217 and a bias circuit 219.
FIGS. 3A to 3C illustrate a conventional TDD switch having a circulator, a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub.
Referring to FIG. 3A, the TDD switch includes an isolator 307, a circulator 301, pin diodes 306 and 304, a λ/4 transmission line 302, a λ/4 transmission line stub 303, and a λ/2 transmission line stub 305. The λ/2 transmission line stub 305 and the pin diode 306 are connected between the isolator 307 and the circulator 301 so as to act as a part of the TDD switch. In addition, the λ/4 transmission line 302, the λ/4 transmission line stub 303, and the pin diode 304 are connected between the circulator 301 and a receive port 310, so as to act as a part of the TDD switch. Also illustrated are a transmit port 308, an antenna port 309 and a bias circuit 311.
In general, a transmission line stub has a specific length and is perpendicularly attached to a transmission line. According to a connection state between the transmission line stub and ground, the transmission line stub may be either an open stub or a shorted stub. Similar to the transmission line, when used in a high frequency circuit, the transmission line stub may also have a characteristic of a specific circuit element. In addition, a λ/4 transmission line has the same characteristic as a λ/4 transmission line stub.
According to the transmission line theory, when the output port of the transmission line stub is not connected to ground (i.e., open stub), the impedance of the input port of the transmission line stub is expressed as Z=−jZo cot βl. Further, when the output port of the transmission line stub is connected to ground (i.e., shorted stub), the impedance of the input port of the transmission line stub is expressed as Z=−jZo tan βl. Here, β=2π/λ, and l is the length of the transmission line stub. As known, waves have the same amplitudes at 0, λ/2, λ, 3λ/2, 2λ, and so on. Hence, the λ/2 transmission line stub 305 may be generalized as a (λ/2)*m transmission line stub [m=0, 1, 2, 3, . . . ]. The λ/2 transmission line stub 305 corresponds to a (λ/2)*m transmission line stub [m=0, 1, 2, 3, . . . ], where m is 1.
The output port of the λ/2 transmission line stub 305 is connected to the pin diode 306. Impedance changes in the pin diode 306 in response to the DC bias allow the output port of the λ/2 transmission line stub 305 to become nearly shorted or open to ground.
When the output port of the λ/2 transmission line stub 305 becomes substantially open to ground, according to the above expressions of Z=−jZo cot βl, β=2π/λ, and l=(λ/2)*m transmission line stub [m=0, 1, 2, 3, . . . ], the impedance Z of the input port of the λ/2 transmission line stub 305 becomes nearly infinite (open-circuited). Since the input port of the λ/2 transmission line stub 305 and a 50 ohm transmission line are connected in parallel to the isolator 307, when the impedance Z of the input port of the λ/2 transmission line stub 305 becomes nearly infinite (open-circuited), the input impedance viewed from the isolator 307 towards the circulator 301 becomes 50 ohm. This is the similar to the case when the λ/2 transmission line stub 305 and the pin diode 306 are not present.
On the other hand, when the output port of the λ/2 transmission line stub 305 becomes substantially shorted to ground, according to the above expressions Z=−jZo tan βl, β=2π/λ, and l=(λ/2)*m transmission line stub [m=0, 1, 2, 3, . . . ], the impedance Z of the input port of the λ/2 transmission line stub 305 becomes nearly 0 (short-circuited).
In the transmission mode, when the TDD control signal is transmitted to the bias circuit 311, the bias circuit 311 supplies a reverse DC bias to the pin diodes 306 and 304. The reverse DC bias allows each of the impedances of the pin diodes 306 and 304 to become nearly infinite (open-circuited).
Since the output port of the λ/2 transmission line stub 305 (nearest to the pin diode 306) located between the isolator 307 and the circulator 301 is connected to the pin diode 306, the impedance of the output port of the λ/2 transmission line stub 305 also becomes nearly infinite. Hence, the output port of the λ/2 transmission line stub 305 becomes substantially open to ground (open-circuited).
Similar to the impedance of the output port of the λ/2 transmission line stub 305, according to the characteristic of the λ/2 transmission line stub 305, the impedance of the input port (nearest to the isolator 307) of the λ/2 transmission line stub 305 becomes nearly infinite (open-circuited). Accordingly, the input impedance viewed from the isolator 307 towards the circulator 301 becomes 50 ohm.
When the TDD control signal operates in the transmission mode, the reverse DC bias allows the impedance of the pin diode 304 located between the circulator 301 and the receive port 310 to become nearly infinite. Since the pin diode 304 is connected to the output port of the λ/4 transmission line stub 303, the impedance of the output port of the λ/4 transmission line stub 303 also becomes nearly infinite (open-circuited). Hence, the output port of the λ/4 transmission line stub 315 becomes substantially open to ground.
The impedance of the input port of the λ/4 transmission line stub 303 is nearly 0, similar to that of the λ/4 transmission line 212 of FIG. 21. The impedance of the output port of the λ/4 transmission line 302 becomes nearly 0 since it is a parallel impedance between 0 and 50 ohm.
The impedance of the input port (nearest to the circulator 301) of the λ/4 transmission line 302 becomes nearly infinite according to the characteristic of the λ/4 transmission line 302. Therefore, it is possible to isolate most of the power signal to be supplied from the circulator 301 to the receive port 310 while the wireless communication system operates in the transmission mode.
Consequently, when the wireless communication system operates in the transmission mode, the λ/2 transmission line stub 305 and the pin diode 306 operate as if they do not exist, and the receive port 310 is protected by the λ/4 transmission line 302, the λ/4 transmission line stub 303, and the pin diode 304. Therefore, the wireless communication system can perform a transmission operation without damaging the receive port 310.
In the reception mode, when the TDD control signal is transmitted to the bias circuit 311, the bias circuit 311 supplies a forward DC bias to the pin diodes 306 and 304. The forward DC bias allows each of the impedances of the pin diodes 306 and 304 to become nearly 0. Since the pin diode 306 located between the isolator 307 and the circulator 301 is connected to the output port of the λ/2 transmission line stub 305, the impedance of the output port of the λ/2 transmission line stub 305 also becomes nearly 0. Hence, the output port of the λ/2 transmission line stub 305 becomes substantially shorted to ground (short-circuited).
Similar to the impedance of the output port of the λ/2 transmission line stub 305, according to the characteristic of the λ/2 transmission line stub 305, the impedance of the input port (nearest to the isolator 307) of the λ/2 transmission line stub 305 becomes nearly 0.
Since the input port of the λ/2 transmission line stub 305 and the 50 ohm transmission line are connected in parallel to the isolator 307, when the impedance Z of the input port of the λ/2 transmission line stub 305 becomes nearly 0, the input impedance viewed from the isolator 307 towards the circulator 301 becomes nearly 0.
When the TDD switch abnormally operates, the TDD wireless communication system may operate in the transmission mode while the TDD switch operates in the reception mode. In this case, the transmission signal amplified through a transmit port 308 is reflected due to impedance changes in the pin diode 306 and is returned to the isolator 307, resulting in termination. Therefore, the circuit of the receive port 310 can be protected.
When the TDD control signal operates in the reception mode, the forward DC bias also allows the impedance of the pin diode 304 located between the circulator 301 and the receive port 310 to become nearly 0. Since the pin diode 304 is connected to the output port of the λ/4 transmission line stub 303, the impedance of the output port of the λ/4 transmission line stub 303 also becomes nearly 0. Therefore, the output port of the λ/4 transmission line stub 303 becomes substantially shorted to ground.
According to the characteristic of the λ/4 transmission line stub 303, the impedance of the input port of the λ/4 transmission line stub 303 changes to be opposite to the impedance of the output port of the λ/4 transmission line stub 303 and thus becomes nearly infinite.
Since the output port of the λ/4 transmission line 302 and the 50 ohm transmission line are connected in parallel to the input port of the λ/4 transmission line stub 303, the impedance of the output port of the λ/4 transmission line 302 becomes 50 ohm. As a result, the impedance of the input port (nearest to the circulator 301) of the λ/4 transmission line 302 becomes 50 ohm according to the characteristic of the λ/4 transmission line 302. This is similar to the case when the circulator 301 is directly connected to the receive port 310.
Consequently, when the wireless communication system operates in the reception mode, according to the operations of the λ/2 transmission line stub 305 and the pin diode 306, the output of the isolator 307 is reflected, and the reflected output is returned to the isolator 307, resulting in termination. Therefore, even if an abnormal output is produced from the isolator 307, the receive port 310 can be protected. In addition, the λ/4 transmission line 302, the λ/4 transmission line stub 303, and the pin diode 304 enable the receive port 310 to smoothly receive a signal transmitted through an antenna port 309.
FIG. 3B illustrates a TDD switch without the λ/2 transmission line stub 305 of FIG. 3A. This is equivalent to the case when m is 0 in a generalized (λ/2)*m transmission line stub [m=0, 1, 2, 3, . . . ]. Other components and operations of the TDD switch of FIG. 3B are the same as those of FIG. 3A. For example, similar to that illustrated in FIG. 3A, FIG. 3B includes an isolator 326, a circulator 321, pin diodes 324 and 325, a λ/4 transmission line 322 and a λ/4 transmission line stub 323. The λ/4 transmission line 322, the λ/4 transmission line stub 323, and the pin diode 324 are connected between the circulator 321 and a receive port 329. Also illustrated are a transmit port 327, an antenna port 328 and a bias circuit 331.
FIG. 3C illustrates the same TDD switch as shown in FIG. 3B except that a λ/4 transmission line 322, a λ/4 transmission line stub 323, and a pin diode 324 of FIG. 3B are connected in two connection configurations. More specifically, the TDD switch of FIG. 3C includes a λ/4 transmission line 352, a λ/4 transmission line stub 353, and pin diodes 354 and 360. About 20 dB of signal attenuation can be prevented per each connection configuration. Thus, the two connection configurations shown in FIG. 3C can prevent about 40 dB of signal attenuation. Other components and operations of the TDD switch of FIG. 3C are the same as those of FIG. 3B. For example, similar to that illustrated in FIG. 3B, FIG. 3C includes an isolator 356, a circulator 351, and pin diode 355. Also illustrated are a transmit port 357, an antenna port 358, a receive port 359 and a bias circuit 361.
As described above, the TDD switch of FIGS. 2A and 2B has a problem in that it cannot be separated by the circulator when errors occur in the antenna port (i.e., a signal is not properly radiated through the antenna). Moreover, when the TDD wireless communication system operates in the transmission mode while the TDD switch operates in the reception mode, a transmission signal may be introduced to the receive port without being blocked by the TDD switch, thereby damaging the circuit of the receive port.
Furthermore, in the TDD switch of FIGS. 2A, 2B, 3A, 3B and 3C, about 0.3 dB of signal attenuation is produced while a signal is transmitted or received, due to the use of the circulator. In addition, the circulator is provided as an additional hardware, thereby increasing a size of the TDD. Accordingly, there is a demand for a TDD switch that can protect the receive port without having to use the circulator when the TDD switch abnormally operates.